A starting frictional engagement element of an automatic transmission is configured by a plurality of input-side and output-side frictional engagement plates arranged offset to each other and a hydraulic piston for displacing these in an engaging direction. When being displaced in the engaging direction by the hydraulic piston, the input-side and output-side frictional engagement plates are pressed against each other to engage the starting frictional engagement element.
When a select lever is operated from a neutral range to a drive range and the starting frictional engagement element is engaged, a hydraulic pressure to be applied to the hydraulic piston is first increased to cause the hydraulic piston to stroke and pre-charging is performed to narrow a clearance between the input-side and output-side frictional engagement plates. A transmission capacity (transmittable torque) of the starting frictional engagement element does not become larger than zero and power transmission is not started before the clearance between the input-side and output-side frictional engagement plates becomes zero.
A time until the transmission capacity of the starting frictional engagement element becomes larger than zero after the select lever is operated from the neutral range to the drive range (hereinafter, referred to as a “start lag”) is preferably constant. Thus, in an automatic transmission in which pre-charging is performed, a learning control of a hydraulic pressure to be applied to a hydraulic piston (hereinafter, referred to as a “pre-charge pressure”) is usually executed so that the start lag is constant (JP 2003-139235A).
In the learning control, the start lag is measured and the pre-charge pressure is corrected to increase since the pre-charge pressure is insufficient if the measured start lag is shorter than a predetermined target time, whereas the pre-charge pressure is corrected to decrease in an opposite case.